Bipolar plating of metal contacts onto oxide interconnection for solid oxide electrochemical cell

ABSTRACT

Disclosed is a method of forming an adherent metal deposit on a conducting layer of a tube sealed at one end. The tube is immersed with the sealed end down into an aqueous solution containing ions of the metal to be deposited. An ionically conducting aqueous fluid is placed inside the tube and a direct current is passed from a cathode inside the tube to an anode outside the tube. Also disclosed is a multi-layered solid oxide fuel cell tube which consists of an inner porous ceramic support tube, a porous air electrode covering the support tube, a non-porous electrolyte covering a portion of the air electrode, a non-porous conducting interconnection covering the remaining portion of the electrode, and a metal deposit on the interconnection.

GOVERNMENT CONTRACT

The Government of the United States of America has rights in thisinvention pursuant to Contract No. DE-AC0280-ET17089 awarded by the U.S.Department of Energy.

BACKGROUND OF THE INVENTION

A solid oxide fuel cell is an electrochemical cell that burns a fuel togenerate heat and electricity. In one embodiment, the fuel cell consistsof a multilayered tube that is electrically connected to other tubes.The electrical connection between the cells is made through a ceramicinterconnection (or a fuel electrode material covering theinterconnection) on one cell and the fuel electrode, which can bemetallic or mixed metallic-ceramic, on an adjacent cell. In between theinterconnection of one cell (or the fuel electrode material covering theinterconnection of one cell) and the fuel electrode of another cell isplaced a spongy nickel felt. The nickel felt permits electrical contactbetween the two cells to be maintained during cell expansion andcontraction which occurs as the cells are heated and cooled. It has beenfound, however, that the electrical connection between the spongy nickelfelt and the interconnection is sometimes poor, which increases theresistance of the cell connections and reduces the efficiency ofconnected cells. If fuel electrode material covers the interconnection,the poor electrical connection is between the interconnection and thefuel electrode material.

Attempts were made to solve this problem by applying a deposit of nickelon top of the interconnection. However, forming the nickel deposit byconventional techniques, such as sputtering or plasma spraying, are notacceptable processes because they are uneconomical or introduce stressesinto the cell structure.

SUMMARY OF THE INVENTION

I have discovered a method of depositing a metal coating on theinterconnection of a solid oxide fuel cell. In the process of thisinvention the coating is electrochemically deposited, with the cathodebeing inside of the cell and the anode being outside the cell.Electrochemical deposition of metal on the interconnection in thismanner is possible because of the particular makeup of the fuel cell.That is, the interconnection is the only exposed portion on the outsideof the cell that is conducting at the deposition temperature, so themetal is deposited only on the interconnection and no masking of otherportions of the cell is necessary. The inner layers of the cell beneaththe interconnection are porous and can be, but need not necessarily be,conducting. Because of these circumstances, ions in a solution insidethe fuel cell tube can carry a charge to the interconnection through theunderyling layers, and metals can be deposited on the interconnectionfrom a solution on the outside of the tube. In this manner I am able toobtain a metal deposit with low resistance and excellent adherence tothe interconnection, so that a good electrical connection can be madebetween the spongy nickel felt or between the interconnection and thefuel electrode material if fuel electrode material is used over themetal deposit. Moreover, this is accomplished even though no electroniccontact is made with the deposit or with the interconnection.

It is somewhat surprising that the method of this invention cansuccessfully produce a good metal deposit because the resistance of theinterconnection material is high, so that it is not a good electronicconductor at the plating temperature (400 to 1,000 ohm-cm). One wouldnot expect to be able to reliably plate electrochemically onto amaterial that is such a poor conductor. But despite the high resistanceof the interconnection at the temperature of deposition, a small currentpasses through the interconnection, while a bipolar potential isimpressed upon it. Also, it is surprising that the material underneaththe interconnection, for example, a modified lanthanum manganite, is notanodically attacked, because it is an anode relative to the cathode thatis placed in the center of the cell tube. Nevertheless, no dissolutionor damage to these underlying layers has been found, which is of greatimportance to the electrical conduction and chemical stability ofmodified lanthanum manganite and, therefore, to the life and performanceof fuel cells (or other electrochemical cells that are equipped withsimilar electrodes).

DESCRIPTION OF THE INVENTION

The accompanying drawing is a top view in section, showing a certainpresently preferred embodiment of two solid oxide fuel cellselectrically connected in series through a metal deposit formed by theprocess of this invention on the interconnection.

In the drawing, a fuel cell 1 consists of a porous ceramic support tube2, generally of stabilized zirconia, over which is a porous airelectrode 3. A portion of the air electrode is covered with a layer ofceramic electrolyte 4 and the remaining portion is covered with theceramic interconnection 5. Covering the electrolyte is thenickel-zirconia cermet fuel electrode 6. A metal deposit 7, according tothis invention, covers interconnection 5 and optional additional fuelelectrode material 8 covers metal deposit 7. A metal felt 9, preferablyof nickel, makes the connection between cells, and between the cells andcurrent collectors 10 and 11.

In operation, a tube is inserted into the center of each fuel cell tubeand a gas containing oxygen is passed through the inside of the tube tothe bottom of the fuel cell tube. Oxygen gas permeates through theporous support tube as it flows between the two tubes. A second gascontaining fuel, such as carbon monoxide, hydrogen, or mixtures thereof,is passed over the outside of the fuel cell tubes. When the oxygenmigrates as an ion through the fuel cell structure it reacts with thefuel, generating heat and electricity. Additional information on thestructure and operation of solid oxide fuel cells can be found in U.S.Pat. Nos. 4,395,468 and 3,400,054, herein incorporated by reference.

In the deposition process of this invention, a fuel cell tube, as shownin the drawing, but without the metal deposit, is immersed in a platingbath. The tube is permanently or temporarily plugged or sealed at oneend so that the plating bath solution outside of the tube, whichcontains ions of the metal to be deposited, does not enter the inside ofthe tube. The inside of the tube is filled with an ionically conductingfluid, and a DC current flowing from a cathode inside the tube to ananode outside the tube electrodeposits the metal on the interconnection.Of course, if it is desired that the metal be deposited on the inside ofthe tank, the positions of the two fluids, and of the anode and thecathode, are simply reversed.

Metals that are suitable for electrodeposition according to the methodof this invention include platinum, gold, copper, nickel, cobalt, andmixtures thereof. Nickel is the preferred metal because the fuelelectrode material presently being used is nickel, and it is notdesirable to form alloys with another metal as that might increase theresistance of the fuel electrode.

The bath from which the metal is deposited is an aqueous solution thatcontains an ion of the metal to be deposited. This solution can beformed from organic or inorganic salts of the above-mentioned metals.Preferably, the anion of the salt should be selected so that itdecomposes at elevated temperatures below about 1000° C. to form gases,as that makes it unnecessary to wash the fuel cell tube after thedeposition has been completed in order to remove any residual salt.Residual carbon can be tolerated because it will be oxidizedautomatically during further processing or cell operation. Examples ofanions that decompose to form gases at elevated temperatures includetartrate, acetate, citrate, nitrate, hydroxyl, and carbonate ions. Forthe same reason, the bath should contain no additives that leaveresidual deposits, such as sulfate, chloride, phosphate, or boratesalts. When such salts, or complex commercial plating solutions withpropietary compositions containing such salts, are used, a prolongedwatering of the cell tube is required. The concentration of the metalsalt in the aqueous solution is not critical, but the preferredconcentration is the concentration having the maximum conductivity, asthis reduces power requirements.

Inside the fuel cell tube is placed an ionically conducting fluid thatcan transport a positive charge through the porous layers of the fuelcell to the interconnection. This fluid is an aqueous solution of acompound that has a low resistance and dissociates into ions in water.Suitable compounds include salts or acids such as ammonium acetate,ammonium tartrate, ammonium citrate, ammonium carbonate, acetic acid,tartartic acid, citric acid, and nitric acid. Salt solutions arepreferred, as strongly acidic or alkaline compounds may attack the airelectrode and damage it. Ammonium salts are particularly preferredbecause they leave no residues when heated and are readily available.The compound selected for use in forming this solution should be onethat decomposes or reacts to form gases when it is heated attemperatures under 1000° C., so that it is not necessary to wash thefuel electrode after the metal has been deposited in order to removeresidual traces of the compound. The concentration of the compound inthe fluid is not critical but it is preferably selected so that thesolution has a maximum conductivity, as that reduces the power requiredfor deposition.

A cathode is placed inside the fuel cell tube in the fluid inside thetube. The cathode may be made of any conducting material includingmetals such as nickel, copper, or iron, but an inert material such asgraphite is preferred.

The anode in the fluid surrounding the fuel cell tube may be either aninert electrode, a sacrificial anode, or the fuel electrode of the fuelcell (6 in the drawing). An inert electrode such as graphite ispreferably not used as the anode because it is then difficult to controlthe volume and concentration of the metal salt in the solution as metalis deposited from the solution onto the interconnection, and freshsolution is added to replace the depleted metal ion. A sacrificial anodeis an anode made of the same metal that is being deposited. It isgradually dissolved as the metal is deposited onto the interconnection.Since the fuel electrode is also made of nickel, the fuel electrodeitself can be also used as the anode. This is especially useful if thefuel electrode is too dense, since its use as an anode increases itsporosity and improves its performance. If the fuel electrode is used asthe anode, the positive terminal of the source of direct current issimply attached to it.

The temperature of deposition may vary, but about 50° to about 70° C. ispreferable for the bath, as lower temperatures may produce a porousdeposit with poor adhesion and lack of uniformity, and highertemperatures are of no additional advantage. The deposition shouldcontinue until the desired deposit thickness is reached. A thickness ofabout 1000 Angstroms to about 20 microns is desirable as a thinnerdeposit may not cover all of the interconnection and a thicker depositmay tend to flake off.

The amperage and voltage of the direct current used are not particularlycritical. A good procedure is to calculate the number of coulombsrequired to deposit a coating of the desired thickness and then adjustthe amperage and time accordingly. The current density should also beadjusted, as is known in the art, to avoid excessive gasing at thedeposit as that may produce deposits that are excessively brittle and/orpoorly adhering.

Once the metal has been deposited on the interconnection of the fuelcell, the fuel electrode type material can be deposited over it. It ispossible to deposit the metal on the interconnection before or after thefuel electrode layer. If fuel electrode material is to be deposited overthe metal or the interconnection, the metal is deposited first, so thatboth the fuel electrode and the fuel electrode material can be depositedin one and the same processing step. A number of fuel cell tubes arethen stacked, as shown in the drawing, to form a fuel cell assembly. Themethod of this invention is applicable to other solid oxideelectrochemical cells, including oxygen gauges, electrolyzers, and gassensors.

The following examples further illustrate this invention.

EXAMPLE

A fuel cell tube as shown in the drawing, but without the metal depositon it, was immersed, sealed end down, in an aqueous bath containing 200grams per liter of nickel acetate. An aqueous solution of 200 grams perliter of ammonium tartrate was poured inside the fuel cell tube. Agraphite rod 6 mm in diameter was placed inside the tube in the ammoniumtartrate solution as the cathode, and two nickel bars 3 mm×8 mm wereplaced in the aqueous bath outside the tube as the sacrificial anode.The temperature of the nickel acetate was 60° C. and the area of theinterconnection was 16 cm². A direct current of 200 mA was applied andthe amperage was gradually increased to 600 mA within one minute. Afterabout 10 minutes the fuel cell tube was removed from the bath andexamined. A strongly adherent deposit of nickel about 1 micron inthickness had formed on the interconnection. The deposit was firmlyattached and covered the entire surface of the interconnection. Fuelelectrode material was deposited on the metal deposit and the tubes werestacked together in series with nickel felt in between to form a fuelcell assembly as shown in the drawing. A stack of three such fuel cellsperformed efficiently with low resistance between the cells for over oneyear.

I claim:
 1. A method of forming an adherent metal deposit on an exposedsurface of an electrically-conducting ceramic comprising:(A) placing anaqueous solution containing ions of said metal in contact with saidsurface; (B) placing an ionically conducting aqueous fluid in contactwith an opposing surface of said ceramic; and (C) passing a directcurrent from a cathode in electrical contact with said aqueous fluid toan anode in electrical contact with said aqueous solution, with noelectronic contact to said deposit or to said surface, whereby said ionsare electrochemically deposited on said surface.
 2. A method accordingto claim 1 wherein said metal is selected from the group consisting ofnickel, cobalt, copper, platinum, silver, gold, and mixtures thereof. 3.A method according to claim 2 wherein said metal is nickel.
 4. A methodaccording to claim 1 wherein the anions of said aqueous solution and theanions and cations of said aqueous fluid solutions decompose to formgases when heated up to 1000° C.
 5. A method according to claim 1wherein said cathode is a graphite rod.
 6. A method according to claim 1wherein said anode is made of the metal to be deposited.
 7. A methodaccording to claim 1 wherein an exposed metal layer on said ceramic issaid anode.
 8. A method according to claim 1 wherein said metal isdeposited at about 50° to about 70° C.
 9. A method according to claim 1wherein said ceramic is in the shape of a tube plugged at one end, andsaid exposed surface is the outside of said tube.
 10. A method accordingto claim 1 wherein said ionically conducting aqueous fluid is a solutionof an ammonium salt.
 11. A method of depositing a metal coating on theelectronically conductive interconnection of a fuel cell tube sealed atone end, where said fuel cell tube consists of an inner porous ceramicsupport tube, a porous air electrode covering said support tube, anon-porous insulating ceramic electrolyte covering part of said airelectrode, an interconnection covering another part of said airelectrode, and a fuel electrode covering said electrolyte that is aninsulator at the temperature that said metal is deposited,comprising:(A) immersing the sealed end of said fuel cell tube into anaqueous solution of a compound dissociated into cations of the metal tobe deposited and anions that decomposes to form gases when heated up to1000° C.; (B) placing inside said tube an ionically conducting aqueoussolution of a compound that decomposes to form gases when heated up to1000° C.; and (C) passing a direct current from a cathode inside saidtube to an anode outside said tube, with no electronic contact to saidcoating or to said interconnection, whereby said cations areelectrochemically deposited on said interconnection.
 12. A methodaccording to claim 11 wherein said deposit is nickel.
 13. A methodaccording to claim 11 including the additional last step of depositingmetal fuel electrode material over said metal coating.
 14. A methodaccording to claim 10 wherein said anode is made of a metal selectedfrom the group consisting of platinum, gold, copper, nickel, cobalt, andmixtures thereof.
 15. A method according to claim 14 wherein said anodeis made of nickel.
 16. A method according to claim 10 wherein saidionically conducting aqueous fluid is a solution of an ammonium salt.